Because Han is supposed to shoot 1st, dammit. Dammit.
Well, at least in the DVD, it seems like they have the same idea at the same time, but Greedo was slightly quicker due to not having to move the gun ever so slightly.
That's my take on the recent stuff.
Personally? As long as Greedo stayed fried, he could have shot first or not at all, I wouldn't care either way.